Lymphocyte development in fish and amphibians

Recently, molecular markers such as recombination activating genes (RAG), t erminal deoxynucleotidyl transferase (TdT), stem cell leukemia hematopoieti c transcription factor (SCL), Ikaros and gata-binding protein (Gata)-family members have been isolated and characterized from key lower vertebrates, a dding to our growing knowledge of lymphopoiesis in ectotherms. In all gnath ostomes there appear to be two main embryonic locations derived from the ea rly mesoderm, both intra- and extraembryonic, which contribute to primitive and definitive hematopoiesis based upon their differential expression of S CL, Gata-1, Gata-2 and myeloblastosis oncogene (c-myb). In teleosts, a uniq ue intraembryonic location for hematopoietic stem cells termed the intermed iate cell mass (ICM) of Oellacher appears to be responsible for primitive o r definitive hematopoiesis depending upon the species being investigated. I n Xenopus, elegant grafting studies in combination with specific molecular markers has led to a better definition of the roles that ventral blood isla nds and dorsal lateral plate play in amphibian hematopoiesis, that of primi tive and definitive lymphopoiesis. After the early embryonic contribution t o hematopoiesis, specialized tissues must assume the role of providing the proper microenvironment for T and B-lymphocyte development from progenitor stem cells. In all gnathostomes, the thymus is the major site for T-cell ma turation as evidenced by strong expression of developmental markers such as Ikaros, Rag and TdT plus expression of T-cell specific markers such as T-c ell receptor beta and lck. In this respect, several zebrafish mutants have provided new insights on the development of the thymopoietic environment. O n the other hand, the sites for B-cell lymphopoiesis are less clear among t he lower vertebrates. In elasmobranchs, the spleen, Leydig's organ and the spiral valve may all contribute to B-cell development, although pre-B cells have yet to be fully addressed in fish. In teleosts, the kidney is undenia bly the major source of B-cell development based upon functional, cellular and molecular indices. Amphibians appear to use several different sites (sp leen, bone marrow and/or kidney) depending upon the species in question.